Trees have long been more than just design elements in urban spaces. Especially in densely built-up areas, they play a vital role in the urban climate: they provide shade, cool their surroundings through evaporation, absorb CO₂ and particulate matter, and create habitats for animals and plants. The City of Vienna therefore even refers to its urban trees as “highly effective, natural air conditioning.” [City of Vienna]
Their significance continues to grow. This is because sealed surfaces made of asphalt, concrete, and stone heat up particularly rapidly during prolonged heat waves. This creates what are known as urban heat islands. Climate change and increasing urbanization will mean that in the future, not only major cities but also smaller towns will be increasingly affected by this effect.
Paris, Vienna, Barcelona
The effort involved in caring for and ensuring the successful growth of urban trees is evident in many European cities. Paris regularly waters young trees during their first three years. As a guideline, the city recommends about 100 liters of water per tree every 15 days from March through September. About 4,000 to 5,000 young trees require special attention during the summer months. For some of these young trees, tensiometers are also used to measure water stress and control watering more precisely.
[https://www.paris.fr/pages/les-jardins-publics-et-les-arbres-proteges-des-fortes-chaleurs-7035]
Vienna, too, invests heavily in maintaining its urban green spaces: Young trees without automatic irrigation are watered by hand once a week during their first four years—and even twice a week during heat waves. To do this, around 150 employees are out on the streets every day with about 50 watering trucks. A total of about 400,000 liters of water is distributed each day —equivalent to roughly 20 average backyard pools with a diameter of 4.5 meters. In addition, nearly 25,000 irrigation bags are used, which slowly release water to the root balls.
[https://www.wien.gv.at/umwelt/stadtbaeume]
Barcelona also relies on targeted water management for its urban green spaces. According to city guidelines, newly planted trees require about 160 to 180 liters of water per watering and are watered regularly during their first few years. At the same time, Barcelona is working to conserve drinking water and is increasingly relying on groundwater and drip irrigation. The city’s strategy during past droughts demonstrates just how important the water supply remains: While irrigation of many green spaces was restricted, essential irrigation for the trees’ survival continued.
[https://ajuntament.barcelona.cat/premsa/2023/03/01/lajuntament-de-barcelona-activara-el-protocol-per-situacio-de-sequera-en-fase-dexcepcionalitat/]
Store water where it’s needed
In addition to targeted irrigation, the question is increasingly being raised as to how the water applied can be kept available in the root zone for as long as possible. One option is water-retaining granules such as Polygrain. They are applied directly to the root zone during planting, absorb water, and store it in the soil. As dry conditions set in, the stored moisture is released back into the immediate vicinity of the roots.
Especially for newly planted urban trees, such an additional water reservoir can be integrated into the planting structure from the very beginning. This allows rainwater and irrigation water to be retained in the root zone for longer and used more efficiently.
But water management doesn’t stop with young trees. Conditions in the root zone of existing and older urban trees can also be improved retroactively. As part of a joint project in Barcelona, water-retaining granules were applied directly to the root zone of existing urban trees using a special lance. This allows the soil’s water-retention capacity to be increased retroactively, keeping water available longer where the tree needs it.
Whether in Paris, Vienna, or Barcelona: These examples show that providing water to urban trees already requires a considerable amount of effort. In addition to efficient irrigation, the question of how water can be better stored in the soil and kept available to trees for as long as possible is therefore becoming increasingly important. After all, investing in urban green spaces today means investing in the cooling and quality of life of our cities tomorrow.
